In recent years, the popularization of the Internet has been rapid with the sources of information becoming more diverse and information capacity increasing. At the same time, the research and development of next generation wireless access systems for implementing high-speed wireless communications in the field of mobile communications have been actively pursued. Next generation wireless access systems include, for example, a HSDPA (High Speed Downlink Packet Access) system which uses Adaptive Modulation and channel Coding (AMC) in which the throughput is determined in accordance with the reception environment of the mobile communication terminals (See FIG. 2). When an HSDPA system is used, the throughput can be rapidly improved. However, in comparison with conventional W-CDMA technology, a mobile communication terminal with more favorable reception sensitivity is required in order to rapidly improve the throughput.
One technology for improving the reception sensitivity of the mobile communication terminal is diversity reception technology which combines and selects signals that are input from a plurality of antennae by mounting a plurality of reception devices in the mobile communication terminal. A technology relating to diversity reception is disclosed in Japanese Patent Application Laid Open No. H8-79146.
Here, with regard to the flow of signals processed in a conventional non-diversity reception mobile communication terminal, the circuit constitution diagram of the mobile communication terminal shown in FIG. 5 will be described hereinbelow. First, the reception signal received by an antenna 81 is amplified while preserving its RF (Radio Frequency) band by an LNA (Low Noise Amplifier) 82. Thereafter, the amplified signal is downconverted by a downconverter 83 and the downconverted signal is subjected to linear amplification by an AGC (Automatic Gain Control) 84. Thereafter, the linearly amplified signal is subjected to quadrature detection by a quadrature detector 85 and the quadrature-detected signal is converted into a digital signal by the A/D converter 86. The digital signal resulting from the conversion is then demodulated by a digital baseband circuitry 90.
On the other hand, in the case of the conventional diversity reception mobile communication terminal shown in FIG. 4, RF receiver circuitries 80A and 80B are provided as a plurality of receiver devices. The RF receiver circuitries 80A and 80B have the same functions as the RF receiver circuitry 80 shown in FIG. 5. The digital baseband circuitry 90 shown in FIG. 4 combines and demodulates digital signals output by the respective RF receiver circuitries 80A and 80B for each antenna.
Further, the power consumption of a conventional diversity reception mobile communication terminal increases in comparison with the power consumption of a conventional non-diversity reception mobile communication terminal. However, when a standby state is used to reduce the power consumption of a conventional diversity reception mobile communication terminal, disconnecting the power supply of another receiver device by activating only one receiver device, that is, disconnecting the diversity reception may be considered. As a method for disconnecting the diversity reception, combining the signals by means of a digital baseband circuitry or performing amplification by means of the LNA of the RF receiver circuitry or the like, for example, may be considered.
However, when the diversity reception is cut expectedly, because the reception SIR (Signal-to-Interference power ratio: a measurement value of the mobile communication terminal that exhibits the reception quality) rapidly deteriorates, it is probable that communications with the base station (including communications with control signals) will be disconnected. This fact will be explained specifically with reference to FIG. 1. First, as shown in FIG. 1A, speech and packet communications are performed between the mobile communication terminal MS and base station BS and data and control signals are exchanged. The SIR of the mobile communication terminal MS in this state is 3 dB. Thereafter, as shown in FIG. 1B, when the diversity reception of the mobile communication terminal MS is unexpectedly cut at the same time that speech and packet communications end, the SIR of the mobile communication terminal MS rapidly deteriorates from 3 dB to −2 dB. When the SIR deteriorates rapidly in this manner, the control signal transmitted from the base station BS can no longer be received by the mobile communication terminal MS, as shown in FIG. 1C.
Further, in a conventional W-CDMA system (in a case where HSDPA is not applied), control is exercised so that the SIR and user throughput of the mobile communication terminal are kept at a fixed level through control of the transmission power by the base station. Therefore, improvements in user throughput are undesirable even when diversity reception is mounted in the mobile communication terminal. In addition, in order to reduce power consumption, disconnecting the diversity reception in HSDPA non-application cells may be considered. Further, in this case, when the mobile communication terminal moves from an HSDPA application cell to a non-application cell (handover), the reception diversity is unexpectedly disconnected. When the diversity reception is unexpectedly disconnected, the combined gain decreases and the SIR rapidly deteriorates. When the SIR rapidly deteriorates, because this cannot be tracked by means of the transmission power control by the base station, the call is disconnected. This will now be explained specifically with reference to FIG. 3. First, as shown in FIG. 3A, when the mobile communication terminal MS is present in an HSDPA application cell C1, the diversity reception operates and the SIR of the mobile communication terminal MS is 3 dB. In this state, the fact that the SIR of the mobile communication terminal MS is 3 dB is reported by the mobile communication terminal MS to both the base station BS1 of HSDPA application cell C1 and the base station BS2 of an HSDPA non-application cell C2. Thereafter, as shown in FIG. 3B, when the mobile communication terminal MS moves from the HSDPA application cell C1 to the HSDPA non-application cell C2, the diversity reception is disconnected. As a result, the SIR of the mobile communication terminal MS rapidly deteriorates from 3 dB to −2 dB. The fact that the SIR is now −2 dB is then reported by the mobile communication terminal MS to the base station BS2 of the HSDPA non-application cell C2. The base station BS2 that detects the fact that the SIR of the mobile communication terminal MS has rapidly deteriorated from 3 dB to −2 dB then disconnects the call of the mobile communication terminal MS.